The answer begs the question, "How do you know that God wrote it?" (Note that if the answer is, "Because it says so in the Bible," then this is an example of circular reasoning.)

Fortunately, if you are speaking to someone who actually knows what they are talking about, their answer to "How do you know that that God wrote it?" will certainly not be guilty of circular reasoning. Their answer should not merely be "Because the bible says so." They should go on to explain that the Bible has been confirmed, and not contradicted, scientifically, archaeologically, historically, and prophetically. The believer should then explain that this makes the Bible trustworthy and, therefore, God wrote the bible.

Grammar note

People often refer to "begging the question" when they really simply mean "raising another question". For example: